1. Field of the Invention
This invention relates to the art of thermal transfer printing or recording and more particularly, to color sheets for the printing.
2. Description of the Prior Art
Broadly, color sheets for thermal transfer printing can be classified into two groups, one group being a thermal fusion ink transfer sheet and the other group being a sublimable dye transfer sheet. With the latter sheet, a large quantity of thermal energy is essential for sublimating or evaporating dyes. In other words, the thermal energy required for the latter sheet is about 4 to 5 times as large as thermal energy for the former sheet. In order to increase a recording or printing speed of thermal fusion ink systems, it will be necessary to use larger thermal energy per unit time than in existing thermal fusion ink systems.
Substrates suitable for use in color sheets should be thin, uniform in quality, resistant to heat and high in mechanical strength. Materials for such substrates should also be inexpensive. The most suitable substrate currently used in thermal fusion ink transfer systems is a polyethylene terephthalate film. This film cannot be used in sublimable dye transfer systems because of poor heat resistance thereof. When polyethylene terephthalate films are used in recording of high thermal energy as in sublimable dye transfer systems, the film tends to attach to a thermal head of the system electrostatically and/or by thermal fusion, causing a so-called sticking phenomenon with the film being finally broken.
In order to solve the above problem, there have been proposed several types of color sheets using specific types of lubricating materials and heat-resistant resins, or specific types of surface active agents of heat-resistant resins.
In these known color sheets, the anti-stick effect can be achieved to an extent with respect to the thermal fusion ink systems. However, satisfactory results cannot be obtained with regard to the sublimable dye transfer systems. Moreover, because of fine irregularities having a size of several micrometers in the heating elements of thermal heads, the resin layer which contacts with the irregular surface of the heating element is gradually scraped off and accumulated on the heating element. The deposit gives rise to the problem that the resulting image has dropouts where white lines or portions are produced.